Molybdenum-base alloys

ABSTRACT

Molybdenum-base alloys are described having a novel crystalline phase structure to provide improved ductility and machinability at room temperature along with improved mechanical properties at elevated temperatures. The alloy has been found particularly suitable for die casting molds to be used in casting ferrous metals as well as non-ferrous metals. The alloy composition has a molybdenum content of at least 90 percent by weight, a nickel content range between 2 and 8 percent by weight, a copper content between 0.5 and 2 percent by weight and may include minor amounts of other alloying metals such as iron, silicon and manganese. A liquid phase sintering method of preparation has been found to optimize the final properties of these alloy compositions.

United States Patent Amra I 1 MOLYBDENUM-BASE ALLOYS [75] Inventor: Lutfi H. Amra, Cleveland Heights, Ohio [73] Assignee: General Electric Company,

Schenectady, N.Y.

[22] Filed: May 10, 1971 [21] Appl. No.: 141,958

[51] Int. Cl. B22f 3/12, B22f 5/00 [58] Field of Search 29/1821, 182;

[56] References Cited UNITED STATES PATENTS 3,418,103 12/1968 Lasdon 75/.5

FOREIGN PATENTS OR APPLICATIONS 531,117 12/1940 Great Britain 497,747 12/1938 Great Britain 521,012 5/1940 Great Britain 517,442 1/1940 Great Britain 760,113 10/1956 Great Britain Primary Examiner--Carl D. Quarforth Assistant Examiner-B. Hunt Attorney-John F. McDevitt, Henry P. Truesdell, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT Molybdenum-base alloys are described having a novel crystalline phase structure to provide improved ductility and machinability at room temperature along with improved mechanical properties at elevated temperatures. The alloy has been found particularly suitable for die casting molds to be used in casting ferrous metals as well as non-ferrous metals. The alloy composition has a molybdenum content of at least 90 percent by weight, a nickel content range between 2 and 8 percent by weight, a copper content between 0.5 and 2 percent by weight and may include minor amounts of other alloying metals such as iron, silicon and manganese. A liquid phase sintering method of preparation has been found to optimize the final properties of these alloy compositions.

1 Claim, No Drawings l MOLYBDENUM-BASE ALLOWS BACKGROUND OF THE INVENTION Various refractory metal alloys are known which possess physical and thermal properties making them useful in various applications. Tungsten alloys have been found particularly useful as lamp filaments for very high temperature operation while molybdenum and its alloys have also been found generally useful for lamp parts operated at lower temperatures. Tungsten alloys have also been used as mold and core materials in the casting of aluminum, brass and even ferrous metals. A ternary alloy of molybdenum containing approximately 0.6 weight percent zirconium and 1.25 weight percent titanium has also been employed heretofore in molds and cores for use in ferrous and non-ferrous casting processes.

A molybdenum-base alloy with physical properties such as low thermal expansion and high thermal conductivity needed for application as a die casting mold has several distinct additional advantages compared with a tungsten-base alloy. The lower density of molybdenum provides a lower cost material of construction in the aforementioned application. Likewise, the easier machinability of molybdenum compared with tungsten provides a further economic advantage for a molybdenum alloy. If tungsten is substituted for molybdenum in a refractory metal alloy composition, on the other hand, there is increased brittleness of the resultant material which is undesirable. Die parts constructed of tungsten-base alloys are further subject to thermal fatigue or heat checking which is generally not experienced as severely with a molybdenum alloy at casting temperatures. Thus, it will be of general benefit to provide a molybdenum-base alloy which has improved physical and mechanical properties for high temperature applications.

SUMMARY OF THE INVENTION Aclass of molybdenum-base alloys having a novel crystalline structure has been discovered which can be machined readily and possesses the desirable mechanical and physical characteristics for high temperature applications, especially in die casting processes. Briefly stated, a new class of molybdenum-base alloys is provided having a crystalline structure which consists of a particulate phase of essentially molybdenum and a matrix phase of a copper and nickel solid solution wherein said alloy has a molybdenum content of at least 90 per cent by weight, a nickel content range between 2 and 8 percent by weight, a copper content between 0.5 and 2 percent by weight, and which may include minor amounts of other alloying metals. Minor amounts of other alloying metallic elements such as iron, silicon and manganese may be added, however, to improve certain desirable properties of the final alloy such as strength, hardness and ductility. Alloys of the present invention also exhibit isotropic mechanical properties which is attributable to a method of preparation that does not include mechanical working of the alloy product such as by extrusion, forging, rolling and the like. In accordance with said method of preparing the alloy compositions of the present invention, a pressed compact is first prepared from a uniform mixture of fine metal particles having the desired final composition, the pressed compact is heated to the sintering temperature for said composition in order to form a liquid phase of the nickel and copper present in the composition having some molybdenum dissolved therein, the heated compact is maintained at the Sintering temperature for a time period sufficient to produce a density of at least percent of the theoretical density for the particular composition, and the sintered compact is cooled rapidly to provide the final product. The pressed compact can be prepared from a uniform mixture of all metallic elements in the alloy composition or from a powdered mixture which includes a prealloy composition of some or all alloying elements being added to finely divided molybdenum powder. Preferred alloy compositions'of the present invention contain 95 percent by weight or more of molybdenum with the balance of the composition comprising primarily nickel and copper in a weight ratio of one or more parts nickel to parts of copper. The weight ratio of nickel to copper in the alloy composition is preferably 2-4- but does not exceed 6.

In the above-described method of preparing alloys of the present invention, it has been found that the sintering treatment given to the pressed compact exerts a sig nificant influence upon the final properties of the alloy obtained. More particularly, too high a Sintering temperature leads to a hard brittle material and can produce a loss of shape or sagging of the sintered product. On the other hand, too low a sintering temperature provides a porous material having inferior mechanical strength and other undesirable properties. Sintering temperatures in the range l,300 C l,400C produce alloys having all the desired physical and thermal characteristics heretofore recited for application as a die casting mold. The time period at which the pressed compact is maintained at the sintering temperature and the cooling rate therefrom have also been found to exert an influence upon the final properties of the alloy product. A time period from 1 to 3 hours at the sintering temperature range above specified provides the desired physical and thermal characteristics. Subsequent cooling of the sintered product at. a relatively fast rate such as can be obtained by quenching or by immediate exposure of said product to the ambient atmosphere provides an alloy having the desirable strength, hardness and ductility for good machinability. When such alloy has been machined into casting molds, it was found possible to obtain several thousand repetitive casting cycles when pouring various type steel compositions without experiencing severe damage of the molds. The present alloys further exhibit a yield strength of at least 100,000 psi at room temperature range and a yield strength of at least 60,000 psi at 540 C. The above reported properties dictate the importance of a proper time-temperature relationship in the method of preparing the present alloys.

In the following description of preferred embodiments for the present invention, all proportions being recited are given in weight percent and the properties being reported were measured by conventional techniques.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The alloy compositions of the present invention all have a two-component structure which consists of a particulate molybdenum component, that is, individual discrete molybdenum crystals in a densely packed relationship which is surrounded by a matrix component whose constituents are essentially copper and nickel in solid solution with molybdenum being dissolved in the solid solution. The molybdenum crystals which constitute the particulate component of the alloy crystalline structure can be further characterized as having a boundary layer of dissolved nickel by reason of the solubility of nickel in molybdenum at the sintering temperatures employed to prepare the alloy. The minimum presence of a matrix component in the alloy crystalline structure serves to point out the absence of porosity in these materials which results in improvement of physical strength and ductility. The close agreement of the presentalloys to the theoretical density is believed attributable to a liquid phase method of preparation whereby a sintering temperature is selected at which the copper, nickel and other alloying elements are liquid and in equilibrium with the solid molybdenum particles.

The above alloy was prepared by starting with a-powdered mixture of the metallic elements in the proportions specified which were blended in a V-cone blender for one hour. The molybdenum powder selected had an average particle size of 3.5 microns. The average particle size of nickel and copper powders used was 2.5 michamber at a travel rate of approximately 13.5 in/hr then removed immediately for cooling by exposure to the ambient atmosphere. The initial presintering treatment has not been found essential in providing an acceptable material but was employed to increase the mechanical strength of the pressed compact sufficiently to withstand the subsequent sintering technique. The alloy product exhibited a density of 9.9 gm/cc along with a Rockwell C hardness in the range 30-34 and a bend angle of 5 at 22 C.

EXAMPLE 2 To illustrate the utilization of different starting materials inpreparing alloy compositions of the present invention, a commercially available prealloy powder containing 29.3 copper, 0.005 carbon, 0.98 silicon, 0.74 manganese, 0.008 sulfur, 0.8l iron and the balance nickel was sifted through a 325 mesh U.S. screen size screen prior to blending with powdered molybdenum. The carbon and sulfur components of said prealloying material are considered at the impurity level hence do not form essential constituents of the final alloy composition. A powder mixture was next prepared having 95 parts by weight molybdenum and 5 parts by weight of the aforementioned prealloy material and pressed at approximately 34,500 psi to form a number of same size compacts for sintering treatment. More particularly, the pressed compacts were all given a presintering step at 800 C to l,000 C for 2 hours in a hydrogen atmosphere then followed by exposure to various sintering temperatures and sintering time periods reported below in Table l. Measurements made-upon the physical properties for each alloy product are also reported in said table.

TABLE I.PROPERTIES OF SINTERED MO-NI-CU ALLOYS Strength (22 0.) Strength (260 0.) Strength (540 C.)

sintering Hardness, 0.2 yield 0.2 yield 0.2 yield Time 'Iem p., Rockwell Density, X1,000 Percent X1,000 Percent X1,000 Percent (hr.) 0. v "C" gm./cc p.s.i. E1 p.s.i. p.s.i. E1

crons. The powdered mixture was placed in a rubber mold and hydropressed at approximately 34,500 psi to provide a pressed compact for sintering. The sintering was accomplished by first heating the compact to l,000 C in a wet hydrogen atmosphere for a 1 hour time period followed by heating to approximately 1 ,4 00 C for an additional 45 minute time period in the same atmosphere to form the crystalline structure previously described. The final sintering was accomplished in a conventional manner wherein the pressed compact was pushed mechanically through the heated furnace It can be noted from the above measurements that a presintering treatment below l ,300 C does not provide a density within 95 percent of the theoretical density which is calculated to be 10.147 gm/cc for the particular alloy composition selected and there is an accompanying loss in ductility as well as mechanical strength of the final alloy.

A further investigation conducted upon the effect of the sintering cycle on the final properties of an alloy having the composition previously recited in this example reportedin Table II below.

The cooling rate expressed in the above table as slow was obtained by leaving the heated samples in the sintering furnace chamber upon completion of the reported sintering time period with the cooling rate being established by ordinary cooling of said chamber upon removal of heat. The cooling rate reported in the table as fast was obtained by removing the samples from the sintering furnace chamber at the end of the sintering time period reported in the table and cooling the samples by exposure to ordinary ambient conditions. The faster cooling rate can be seen to improve ductility of the final product which renders the material more generally useful.

It will be apparent from the foregoing description that a new class of molybdenum-base alloys has been provided which are useful as molds and cores for die casting and other applications. It should also be appreciated from the foregoing description that alloys of the present invention can be prepared which may include minor amounts of still other alloying metalsthan above disclosed but still having comparable final properties so that it is intended to limit the present invention, therefore, only to the scope of the following claims.

I claim:

ll. A casting die fabricated from an alloy having the composition consisting essentially of a particulate phase of molybdenum having dissolved nickel, with a matrix phase of a copper and nickel solid solution having dissolved molybdenum, wherein said alloy has a molybdenum content of at least percent by weight, a nickel content between 2 and 8 percent by weight, a copper content between 0.5 and 2 percent by weight and which may include minor amounts of other alloying metals.

l l 8 1 l 

